Semaphorin 4D (SEMA4D), also known as CD100, is a transmembrane protein (e.g., SEQ ID NO: 1 (human); SEQ ID NO: 2 (murine)) that belongs to the semaphorin gene family. SEMA4D is expressed on the cell surface as a homodimer, but upon cell activation SEMA4D can be released from the cell surface via proteolytic cleavage to generate sSEMA4D, a soluble form of the protein, which is also biologically active. See Suzuki et al., Nature Rev. Immunol. 3:159-167 (2003); Kikutani et al., Nature Immunol. 9:17-23 (2008).
Immunohistochemical analysis of SEMA4D in a large tumor sample collection revealed that SEMA4D overexpression is a very frequent event in head and neck, prostate, ovarian, pancreatic, colon, breast, and lung cancers as well as being significantly expressed in other tumor types. SEMA4D is a potent pro-angiogenic molecule. Activation through a SEMA4D receptor, Plexin-B1 promotes angiogenesis both in vitro and in vivo. See, e.g., Sierra JR, et al. J Exp Med 205:1673-1685 (2008). Plexin-B1 is referred to herein and in the scientific literature interchangeably as, Plexin-B1, plexin-B1, Plexin B1 or plexin B1.
The angiogenic response elicited by SEMA4D is comparable to that elicited by other angiogenic molecules, such as vascular endothelial growth factor (VEGF). It is well established that VEGF and its receptors are key regulators of new blood vessel formation, or angiogenesis. The VEGF gene family has several members, including VEGF-A (also referred to herein as “VEGF”), VEGF-B, VEGF-C, VEGF-D, VEGF-E, and P1GF. See, Ho and Kuo, Int. J. Biochem Cell Biol. 2007; 39(7-8): 1349-1357. There are numerous alternatively spliced isoforms of human VEGF, including VEGF165, VEGF121, VEGF189, and VEGF206. See Ho and Kuo, Int. J. Biochem Cell Biol. 2007; 39(7-8): 1349-1357; Ferrara et al., Nature Med. 2003; 9 (6):669-676. The VEGF165 isoform is the most prevalent and mitogenic and is most similar in properties to the 45 kDa native VEGF. See Ho and Kuo, Int. J. Biochem Cell Biol. 2007; 39(7-8): 1349-1357; Ferrara et al., Nature Med. 2003; 9 (6):669-676.
VEGF exists as a 45 kD homodimeric glycoprotein which binds to two related tyrosine kinase receptors. Ferrara et al., Nature Med. 2003; 9 (6):669-676. VEGFR-1 (also known as Flt-1), is a high affinity receptor for VEGF whose function is not fully understood. VEGFR-2 (also known as KDR or Flk1), is the other high affinity receptor for VEGF, and is the receptor through which the pro-angiogeneic activity of VEGF is believed to be induced. See Ferrara et al., Nature Med. 2003; 9 (6):669-676. VEGFR-2 forms a dimer and autophosphorylates when bound to VEGF. Dougher and Terman, Oncogene. 1999; 18: 1619-1627. This, in turn, activates several signaling cascades that promote endothelial cell growth and migration, and which, ultimately, lead to angiogenesis. See Hicklin and Ellis, J. Clin. Oncol. 2005; 23(5): 1011-1027.
During embryonic and postnatal development, VEGF participates in angiogenesis, vasculogenesis, and lymphangiogenesis. VEGF has also been found to play a role in adult processes, as well, including ovarian angiogenesis, endochondral bone formation, tissue regeneration, survival of hematopoietic stem cells, and regulation of erythropoietin. See, Ho and Kuo, Int. J. Biochem Cell Biol. 2007; 39(7-8): 1349-1357. It is the involvement of VEGF in disease processes such as cancer and other neoplastic conditions, inflammatory disease, ocular disease, and ischemic disease that makes it a potential target for treatment.
Angiogenesis is a requirement for a tumor to grow beyond 1 to 2 mm. The formation of new vasculature is a result of the tumor environment “switching” on several pathways that promote tumor angiogenesis. Inhibition of VEGF-induced angiogenesis by a monoclonal antibody that specifically binds to VEGF was shown to suppress tumor growth in vivo. See Kim et al., Nature 1993; 362: 841-844. This observation was the motivation for development of a therapeutic antibody, bevacizumab, to neutralize VEGF and slow the progression of cancer.
There is significant toxicity associated with clinical use of ant-VEGF antibodies. There remains, therefore, a need for cancer treatments, and in particular therapeutics that inhibit, suppress, prevent, slow the progression of, shrink, or directly attack angiogenesis.